Modern equipment which use radio waves for communications, such as cellular telephones, generally operate at very high frequencies. These devices use frequency synthesizers to produce the high frequencies required. However, crystals are still needed for the high stability frequency they produce. Frequency synthesizers, such as phase locked loop (PLL) frequency synthesizers phase lock with the highly stable signals of crystals to produce stable high frequency signals.
Essentially, frequency synthesizers, especially PLL frequency synthesizers, phase lock with a crystal signal and multiplies the frequency of that crystal signal to produce a utilizable high frequency stable signal. Thus, the high frequency signal used by the radio equipment is a multiple of the crystal frequency. It should be noted that the multipliers used by frequency synthesizers are not limited to integers. Fractional-N frequency synthesizers can use fractional numbers as multipliers. This feature allows highly accurate frequencies to be produced.
During production, radios have to be tuned to perform at expected frequencies. However, tuning radios to the proper frequency can be time consuming and expensive. Frequently, radios have a tunable component which is adjusted to obtain the correct frequency by a technician during production.
One problem with crystals is that, under certain conditions, the frequency of their signals tends to drift away from the expected frequency. The phase of the crystal signal is still stable; however, the frequency may now be different. Temperature, age of the crystal, and other known factors can cause this unfortunate phenomenon. When this occurs, given that the high frequency signal generated by the frequency synthesizer is a multiple of the crystal frequency, the high frequency signal drifts as well. This can lead to poor transmission and/or reception between radios as the frequency being used is no longer the desired frequency. To remedy this problem, periodic calibration of radios is performed. This entrails adjusting the crystal frequency to obtain the correct high frequency signal. Clearly, such methods can be expensive, requiring time consuming disassembly of radios and shop time for technicians to perform the calibration.
Another measure currently in use to compensate for frequency drift in crystals is the use of Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO). Using a VCTCXO, the new frequency after drift is determined to be higher or lower than the desired frequency. After this determination, a correction voltage is sent to the VCTCXO to compensate for the drift. However, for the most part, radio manufacturers allow their crystals to drift without correction or compensation. To minimize the possibility of such drifts, manufacturers can use more expensive crystals which are either less prone to drift or which do not drift as much.
Another problem with radios concerns the well known and well documented Doppler effect. Given two communicating radios, with at least one radio in motion relative to the other, the transmission frequency perceived by either radio from the other is constantly changing. This frequency varies as the velocity between the two radios changes. Clearly, the changing frequency perceived by either of the two radios makes for problematic transmissions.
What is therefore needed is a method and an apparatus that allows for fast and inexpensive calibrations of radios. Also needed is a method which compensates for Doppler effects.